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Near Surface 2008 - 14th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 15 Sep 2008 - 17 Sep 2008
- Location: Kraków, Poland
- ISBN: 978-90-73781-56-6
- Published: 17 September 2008
21 - 40 of 130 results
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Geophysical Survey of Salpausselkä Glacial Deposit near by the City of Hanko in Finland
Authors T.M. Hokkanen, H. Rainio and E.L. LaineIn Finland we have lots of esker and end moraine deposits originated from the time of glacial period and from the time of glacial rebound. The purpose of this study is to investigate those deposits with different geophysical methods. We carried out ground penetrating radar, refraction seismic, and resistivity imaging measurements when investigating Salpaussekä end moraine near by the city of Hanko in Southern Finland. Moreover, we bored four reference boreholes and exploited all geological data in available when making the final interpretation. As a result of these studies, we identified different soil types and the topography of bedrock. We are also able to evaluate better the genesis of certain parts of Salpausselkä end moraine in the vicinity of the city of Hanko.
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Three Step Depth Focussed Inversion as a Tool to Resolve Small Resistivity Contrasts by ERT
Authors U. Noell, T. Günther and M. FurcheIn more recent times Electrical Resistivity Tomography (ERT) has become a valuable tool for many environmental research themes. Within these topics the resistivity contrasts of the different layers and zones are of importance, i.e. sand, clay, water salinity and preferential infiltration pathways are quite small. Inclusions with such small resistivity contrasts to the surrounding are difficult to resolve by ERT inversion and typically the following problems occur: 1.) the true resistivity contrasts are underestimated 2.) the size of the anomalous zone is overestimated 4.) “fake” anomalies arise beside and between the true anomalous zones. 4.) surface heterogeneities infer “fake” anomalies into the subsurface. A two step inversion method is suggested to overcome these problems at least partly. Firstly the near surface heterogeneities are reconstructed by inversion. A forward model comprising the near surface structures only is extracted and used to remove their effect from the data by the application of a reference inversion method. Forward modelling is used to estimate the true sizes and the true resistivity contrasts of the inclusions. These methods are applied to array electrical resistivity tomography measurements at a sandy site with distinct heterogeneities at a depth of 30 – 60cm and show improved resolution capabilities.
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Dynamic Dilatancy in Saturated Sands
Authors E.S. Kushnareva and E.A VoznesenskyNew experimental aspects of saturated sands dynamic response are discussed. It is demonstrated that shear straining in saturated sands with any density occurs in several stages. Incremental changes of pore pressure and effective stresses at the shear zones triggering point are determined by initial sand porosity relative to its critical value. Sand porosity before shearing is considered as critical, if triggering of shear zones does not result in porosity alteration, which in undrained conditions is physically equivalent to zero change of mean effective stress and pore pressure. Type of sand response is revealed as incremental changes of pore pressure and mean effective stress at the stage of shear zones triggering, indicating negative or positive soil dilatancy. The notion of critical porosity is not applicable in dynamic loading conditions since in different phases of a single cycle changes of pore pressure and effective stress are observed. Progressive strain accumulation under dynamic loading of saturated sands with any density results in the development of a specific state with zero sum changes of pore pressure and mean effective stress called zero dynamic dilatancy. It is assumed to relate to the development of shear zones with constant thickness where subsequent straining is further located/
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Petrophysical Analysis of Resistivity Data
Authors V.A. Shevnin, A.A. Mousatov, A.A. Ryjov and O. Delgado-RodriguezRyjov developed forward problem of soil resistivity for sand-clay models. It uses 9 parameters (clay and sand porosity and capillary radii, water salinity and humidity, cation exchange capacity, temperature and clay content) to calculate soil resistivity. We applied these calculations for petrophysical analysis of sand clay soils using soil resistivity obtained from VES interpretation. It became clear that dependence of resistivity from clay content, porosity, cation exchange capacity, etc. allows solving inverse problem (estimation of petrophysical parameters on soil resistivity), that was fulfilled as algorithm and software in 2003. It is evident that we can’t determine all 9 parameters by using only soil resistivity; we need to find and use some additional information to have quantity of known parameters more than unknown ones. Temperature and groundwater salinity can be determined in each field site. We take soil samples to measure soil resistivity versus pore water salinity in laboratory to obtain soil model and recalculate resistivity data (cross-sections and maps) into cross-sections and maps of petrophysical parameters: clay content, porosity, cation exchange capacity, filtration coefficient, etc. After 2003 this technology was probed at more than 20 field sites in Mexico and Russia and demonstrated its efficiency.
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A Study of Hillslope Hydrologic Dynamics Using Irrigation Tests and Time-lapse 3D Electrical Resitivity Tomography
Authors G. Cassiani, S. Stocco, A. Godio, A. Villa, P. Frattini, M. Rossi, R. Deiana and G.B. CrostaThe description of catchment response is a fundamental issues in geoscience. However, some of the basic governing processes, particularly the role of the subsurface, remain poorly understood. In this paper, we present the results of non invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Alps. The key hydrologic question is whether rainfall infiltrates mainly into the underlying bedrock or flows in the soil layer towards the stream channel a few hundred meters downgradient. We applied 1000 mm of artificial rain on a 3 m x 3 m slope box over about 18 hours. The soil moisture variation and the underlying bedrock was monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT was performed via 2 m deep borehole purposely drilled and completed with electrodes in the irrigated plot. As a result, we observed a fast vertical infiltration through the soil cover followed by infiltration into the fractured bedrock. The lateral transmission of infiltrating water was negligible. This experiment confirms that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study.
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Joint Interpretation of Magnetic Resonance Sounding and Borehole NMR Data
Authors TH. Hiller, U. Yaramanci and J.P. DelhommeThe main objective of the joint research project between the Technical University of Berlin and Schlumberger Water Services was to assess how high resolution borehole NMR can be used to enhance the interpretation of Magnetic Resonance Sounding (MRS) measurements. Therefore a profile measurement on a ground water test site in the desert of Abu Dhabi was carried out. With new and extended processing and inversion techniques we were able to interpret the well near MRS soundings together with the information from the borehole. On the one hand the borehole data was used to directly improve the inversion by implementing the resistivity-log data. On the other hand we could verify the results of our surface measurements when comparing the inverted water content and the belonging decay times with the equivalent CMR Bin porosity classes from the borehole.
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Resolution Limits of NMR-based Estimation of Pore Size Distributions
Authors S. Strehl and U. YaramanciNuclear Magnetic Resonance (NMR) methods offer the determination of pore size distributions of loose sediments, which is the basis of investigations concerning ground water dynamics, i.e. water content and hydraulic conductivity of the subsurface. The NMR resolution range of pore radii depends on many parameters such as the technology of the applied NMR system, the Larmor frequency and the properties of the pore fluid and the pore surface, respectively. Different NMR methods do not necessarily show the same results at the same samples. At core samples from a shallow aquifer we compare Earth’s field NMR measurements with conventional lab NMR measurements and show the influence of the dead time and the Larmor frequency on the resolution limits of the pore size distribution.
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Well-logging and Near-surface Seismic Methods for Aquifer Detection
Authors G.M. Suarez, S. Miong, J. Wong, R.R. Stewart, A.D. Alcudia, H. Lu and K. Al DulaijanWell-logs, high resolution shallow seismic, and VSP surveys were conducted in a experimental well near Priddis, Alberta, to test their feasibility in characterizing the near-surface stratigraphy and hydrogeological characteristics of the Paskapoo Formation. According to the analysis of well logs, the well penetrates interbeds of sandstone-shale layers and five porous zones were identified at depths of 28, 39, 50, 62 and 120 m, ranging from 0.34 to 0.58. Among these porous zones, there were three water-bearing zones at depths of 28, 50 and 120 m. Based on the P-wave velocities calculated from the sonic log, surface-wave analysis was carried out to estimate S-wave velocities. Up to depths of 40 m, the respective P-wave velocities for the sandstone and shale units were 2250 m/s and 2080 m/s, and 1220 and 1150 m/s for the S-wave, respectively. Using a microphone-geophone analysis, the airwave was characterized by high frequency and broadband. The integral interpretation of all the datasets and well-logs show a good correlation between all of them. The water-bearing zones (28 and 50 m) and the strong reflector at 76 m were resolved by the surface seismic and VSP data.
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Ground Water Detection by Means of Electrical and EM Methods – The Case of Paliouri, Northern Greece
Authors G.N. Vargemezis, P. Tsourlos, A. Stampolidis, I. Fikos and D. BallasIn this study the resuls of the application of electrical methods (Electrical Resistivity Tomography and Self Potential) and VLF method are presented. Since underground water resources were expected mainly in faulting zones of the ophiolithic outcrops, initially the VLF method has been applied in order to detect faulting zones. In a second phase SP and ERT measurements have been conducted in order to further study the characteristics of the faults. Finally, positions for the wells to be drilled have been proposed.
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Photometric Flowmeter for Measuring Slow Vertical Water Flow in Wells
Authors J. Urík, S. Mareš, M. Kobr and S. RezaninaThe logging tool is equipped with injector enabling to create in the streaming water a colour mark using the food dye Brilliant Blue (E155) as a tracer. A pair of photometric sensors is located in the axis of the flowmeter in a distance 5 and 10 cm above and below the special outlet nozzle of the injector. The probe is capable to measure vertical flow velocities in the range 10E-4 to 10E-2 m/s or vertical flow rates in the range 10E-6 to 10E-4 m3/s. The proper function and reliability of the logging tool were checked in the model of the well under laboratory conditions.
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Recovering Old Geophysical Documents for Deep Aquifer Research
Authors J.L. Plata, M. Riveira and P. IbarraMost of the geophysical field data do not depend on the objectives and date of their measurement, and can be used again with another purpose, whenever its quality and the acquisition parameters allow it. The Geological Survey of Spain (IGME) is involved in the development a new database gathering most of the geophysical surveys carried out within the institution in the last thirty-forty years and is now incorporating data from other sources as the hydrocarbons exploration. For ancient surveys the most normal situation is that original field data no longer exists, and only paper copy documents are available. Two examples of recovering such information are given. They belong to multidisciplinary projects for the development of a methodology for hydrogeological research. In both cases the reprocessing and/or reinterpretation of old documents has provided fundamental information and served as an excellent guide for optimizing the design of new geophysical surveys.
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Quantitative Appraisal of Noise in Time Domain Induced Polarization Data
Authors A. Viezzoli, J. Pedersen and A. PytlichThe possibility to acquire rapidly time domain IP data with multi channel systems is spreading the application of this technique. The quality of such IP data measured in the field is an important, yet not well documented matter. We present a systematic study of noise levels and sources in time domain IP data, carried out with repeated measurements, and different instrumental set ups, in two distinct geological settings. The results show that relative (random) noise to signal values are usually in excess of 10 % at early times, higher at late times. Measuring too early after placing the (stainless steel) electrodes in the ground seems to affect the early times of the decay. Periodic sub harmonics of the 50 Hz can be found in some occasions superimposed to the IP data. It is very important to obtain good electrical contact between electrodes and ground in order to lower both random and coherent noise, in particular capacitive coupling. Inductive coupling is reduced to the minimum using separated cables and electrodes for sending the current and measuring the potential. Full analysis of the whole IP decay curve is crucial for assessing data quality.
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Detection of Hydrocarbon Contamination with 3-D Resistivity and IP Method
Authors B. Neducza and E. TörösThe abstract shows a case history of a 3-D geoelectrical measurement on a contaminated site. It illustrates the advantage of IP measurement with non-polarizable elecrodes furthermore a resistivity/chargeability crossplot is shown for quality control prior to inversion.
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Integrated Interpretation of Geophysical Data on Metalliferous Mining Waste Deposits
More LessThe geophysical investigation of waste dumps has registered a huge progress in the last two decades, benefiting implicitly of a significant improving until present times of the data acquisition, processing and interpretation techniques. The integrated interpretation of many datasets, each belonging to a different investigation method, proved to be the optimum solution for high quality results in the condition of relatively small investments in the data acquisition stage. The resistivity, conductivity and magnetic surveys are some of the cheapest near-surface investigation methods. In this study we present an approach of the integrated investigation of metalliferous mining waste deposits using these three methods.
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Geophysical Imaging of a Tailings Lagoon at an Abandoned Lead-zinc Mine in the Central Wales Orefield, UK
Authors O. Kuras, V. Banks, B. Palumbo-Roe and B. KlinckAs part of ongoing research into the environmental impact of abandoned metalliferous mines in the Central Wales Orefield in the UK, a geophysical study was undertaken in order to characterise mine tailings at the Frongoch Mine near Devil’s Bridge, Ceredigion. A combination of ERT and GPR surveys over a tailings lagoon has revealed the extent of the tailings deposits as well as the presence of underlying strata, including peat and glacial till. Observed resistivities are in accordance with the properties of samples taken from trial pits and with the previous extent of the lagoon indicated on historic maps. It has been demonstrated that GPR can successfully map peat and glacial till layers below the conductive tailings deposits. Under the circumstances, GPR data proved more useful to estimate the thickness of the tailings layer, while ERT may hold more information about superficial cover and bedrock topography. The potential of non-invasive geophysical techniques as a rapid and cost-effective tool in the context of the appraisal and environmental impact assessment of mine waste has been highlighted.
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Observations of the Bioremediation of Oil Contaminated Soils – Application of EM and Resistivity Imaging Methods
Authors B. Zogala, R. Dubiel, M. Robak, W.M. Zuberek and K. WzientekThe survey has been carried out in the area the former military underground fuel base situated in the firing ground Borne Sulinowo (NW Poland). The oil contaminations were revealed in 7 zones in the studied area. In 2003 and 2005 yeast strain Yarrowia lipolytica A-101 suspensions were introduced into shallow bore-holes to intensify the bioremediation process. In succeeding seasons the bioremediation was observed and monitored using EM and resistivity imaging measurements. The study showed the significant measurable change in soil electrical properties after several months since the yeast suspension had been introduced into hydrocarbons contaminated soil. The bioremediation can be efficiently observed and monitored with geoelectrical methods such as resistivity imaging or electromagnetic induction ones.
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Potential of Geoelectrical Imaging Techniques for Detecting Subsurface Gas Migration in Landfills – An Experiment
Authors T. Dahlin, H. Rosqvist, V. Leroux, M. Svensson, M. Lindsjö, C.H. Månsson and S. JohanssonWe measured eleven regularly spaced resistivity profiles over a ten by ten meter experimental plot where high landfill gas (LFG) emissions had been recorded on the surface. The measurements were repeated seven times during four days which made it possible to trace the development of resistive areas. The LFG emissions at the surface of the landfill were estimated with laser absorption spectroscopy and static chamber measurements at the same time as resistivity surveying. By studying the changes of resistivity with time it was indicated that the gas migration in the waste mass was a relatively fast process, changing within only a few hours. Our presentation will focus on details regarding temporal and spatial changes of measurement.
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Airborne Geophysical Studies around Landfill Areas – Case Histories from Finland
Authors J. Lohva, T. Jokinen and I. SuppalaGeological Survey of Finland (GTK) had completed an airborne low altitude geophysical mapping survey covering the whole Finland. In recent years airborne data have been used in environmental applications related to landfill, groundwater and soil contamination. In this paper we present aerogeophysical studies of landfill areas in Finland. The first examples are from closed landfills, Mäntyvaara in north Finland and three landfills in Helsinki region. Mäntyvaara landfill was the first case in Finland, where AEM data were used for mapping contaminated areas. The other examples are monitoring cases. The airborne measurements have been conducted four times over the Ämmässuo and twice over the Metsä-Sairila landfills. Using GTK's three-in-one airborne system to locate possible leakages is a useful method. The AEM out-of-phase component is sensitive to detect small changes in conductivity. AEM modelling and inversion are useful tools for estimating detailed conductivity changes. Magnetic data can be used to interpret linear features of bedrock to detect possible path of leakages. Radiometric data can estimate soil thicknesses and the changes of moisture. The changes in the environment can be observed more reliably from natural condition if repeated measurements are carried out.
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Geophysical Investigations of a Chemical Munition Dumpsite in the Baltic Sea
By T. MissiaenVery high resolution seismic and magnetic investigations were carried out over a chemical munition dumpsite in the south-western Baltic Sea. The shallow geology was imaged in great detail on the seismic data and seven seismic-stratigraphic units were identified, related to different stages in the Holocene and late-glacial history. A large number of diapir-like features were observed that most likely represent fluid expulsion phenomena. Seismic and magnetic data indicate the presence of a large number of buried objects. In most cases there is a good correlation between the two data sets. The objects are buried no deeper than 1-2 m. Their size varies between 1.5 and 5 m, occasionally up to 10 m. The data confirm the wide variety of dumped war material ranging from bombs and shells to encasements and containers. The distribution of the buried objects seems heterogeneous with locally high object concentrations surrounded by areas of lower object density. The results of this case study demonstrate the benefit of complementary, concurrent geophysical investigations for munition dumpsite research. This will yield a better assessment of the current status of the dumpsite and possible ecological risks related to the dumped war material.
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Monitoring Changes in Bentonite at the Grimsel Test Site Using Crosshole Seismic Tomography
Authors S. Marelli, H.R. Maurer, E. Manukyan, S.A. Greenhalgh and A.G. GreenWe have been exploring the feasibility of monitoring the swelling of bentonite using non-intrusive crosshole seismic tomography. Our study has included numerical simulations and extensive field experiments at the Grimsel Test Site in the central Swiss Alps. Numerical simulations have demonstrated that changes in bentonite's elastic properties resulting from variations in water-saturation conditions should yield measurable effects in crosshole seismic data. Based on the simulation results, a relatively thin block of encased bentonite at the end of a tunnel was surrounded by six inclined boreholes. Seismic energy sequentially generated in three of the holes was detected by hydrophones in the other holes. The recorded seismic wavefields, characterized by high signal-to-ambient-noise ratios and coherent frequencies of 1-4kHz over propagation distances exceeding 20m, have provided sufficient information for meaningful high resolution tomographic inversions. Varying the water-saturation conditions resulted in marked changes in the transmitted and reflected wavefields that were readily observable in the raw data. Source and receiver coupling effects also varied significantly during the course of our experiments, and unusual phases due to wave propagation in the source and receiver cables and to borehole-related phenomena were prominent features of the seismic sections.
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